1. Field of the Invention
The invention relates to a process for recovering sodium nitrite, potassium nitrite, lithium nitrite, or nitrite mixtures containing at least one of the foregoing from the reaction mixture resulting from the synthesis of a polyimide such as, for example, an aromatic bis(ether phthalimide).
2. Brief Description of the Related Art
Sodium nitrite may be produced as a reaction by-product in aromatic displacement reactions such as the synthesis of aromatic bis(ether phthalimide) compounds. Synthesis of these compounds has been described in U.S. Pat. No. 5,068,353 to Dellacoletta. Of particular interest is the product of sodium nitrite as the reaction by-product of the synthesis of bisimide having the formula (I). ##STR1##
Several techniques have been used to recover bisimide from a reaction mixture including solid-liquid separation techniques such as filtering at a temperature at which the bisimide is substantially completely soluble while alkali metal salt impurities are substantially insoluble. (See U.S. Pat. No. 5,068,353.)
Bisimide has also been recovered by extractive purification employing a conventional caustic wash as the extractant. In this process, bisimide in the toluene reaction solvent is extracted with a sodium hydroxide wash to remove the sodium nitrite, unreacted starting materials, catalyst and other reaction by-products. A disadvantage to this method is that the sodium hydroxide also hydrolyzes some of the bisimide product, converting it to aqueous, soluble amide-acid sodium salts. (See U.S. Pat. No. 5,068,353.)
The sodium hydroxide wash, containing the bulk of the sodium nitrite present in the reaction mixture, is typically disposed of by concentrating and burning in an incinerator or is disposed of through biotreatment. The organic materials in the wastewater (i.e., the sodium hydroxide wash) are destroyed and the sodium nitrite is converted to nitrogen and sodium carbonate in the burning process. A disadvantage to this process is the violent, uncontrollable nature of the reaction due to the high amounts of organic impurities present in the sodium hydroxide wash. As the sodium nitrite by-product can itself be marketed, it would be advantageous to recover the sodium nitrite from the reaction mixture in sufficient purity to be marketable.
What is needed in the art is a method for recovering useable sodium nitrite from the reaction mixture formed from the synthesis of aromatic bis(ether phthalimides).